Optical coherence tomography (OCT), sometimes referred to as “optical biopsy”, can be used to obtain high-resolution (˜10 μm) cross-sectional imaging of scattering biological tissues up to 3 mm deep. OCT is based on low-coherence interferometery and fiber optic technology. The core of an OCT system is a Michelson interferometer, a simplified schematic of which is shown in FIG. 1. For a typical fiberoptic OCT imaging system, such as system 100 shown in FIG. 1, two optical fibers after the beam splitter 110 are required. A first optical fiber (fiber 1) is used for the reference arm of the interferometer, while a second optical fiber (fiber 2) is used for the sample arm of the interferometer which scans the sample 120. The reference arm is external to the probe, while the sample arm including fiber 2 and the sample arm optical components are embedded inside the imaging probe, such as within a catheter for insertion into the body cavity of a patient. For cardiovascular imaging and endoscopic imaging, slender catheters are required. Accordingly, the OCT must be constructed as a slender imaging probe.
Optical interference is detected by the photodetector 125 only when the optical path difference between the reference and sample arms is within the coherence length of the light source 130. Depth information of the sample is acquired through the axial scanning (z) of the optical delay line provided by reference mirror 135 in the reference arm. Two-dimensional (2D, i.e., x-z) cross-sectional images are obtained by a 1D (or 2D) transversely scanning mirror 140. 3D images can also be obtained if a 2D transversely x-y scanning mirror is provided.
The axial resolution is determined by the coherence length. Low coherence is obtained by using a broadband light source such as a superluminescent diode (SLD) or a femtosecond laser. The coherence length of a broadband light source is given by 0.44 λ2/Δλ, where λ and Δλ are respectively the center wavelength and spectral bandwidth of the light source. For example, a SLD with a center wavelength of 1300 nm and a bandwidth of 90 nm has a coherence length of 8 μm. Thus, OCT imaging can achieve at least one order of magnitude higher spatial resolution compared to commonly used ultrasound imaging (˜100 μm). Furthermore, study shows that more than 85% of all cancers originate in the epithelial layer which is within the penetration depth of infrared laser beams. Thus, OCT can be used for cancer diagnosis and has been applied to a wide variety of biological tissue and organ systems including eyes, skin, teeth, gastrointestinal tracts and respiratory tracts. OCT can also be used for cardiovascular imaging. Cancer and heart disease are the top two killers in U.S. and most of the developed world.
OCT provides high-resolution cross-sectional images, which is suitable for early cancer diagnostics and plaque detection in coronary arteries. Conventional OCT obtains image data pixel by pixel. Each pixel corresponds to an axial scan and a lateral scan. The light beam focus size thus determines the image resolution. Full-field OCT systems have been reported to overcome some of these problems. In full-field OCT systems, a two-dimensional (2D) image is obtained for each axial scan without any lateral scan. As a result, images can be generated much faster in comparison to conventional OCT. Furthermore, high resolution can be obtained by using a large array of photodetectors, not limited by the light beam spot size. However, current full-field OCT systems require the Michelson interferometer formed in free space to provide spatial and phase correlations. Thus they are bulky and are accordingly limited biopsy samples, or for external use.